High intensity discharge (HID) arc lamps are in wide use for general illumination. Applications include roadside street lamps, sports arena illumination, stadium illumination, auto dealership illumination, warehouse illumination, and other purposes requiring a high power of illumination with high efficiency. They tend to be mounted at fairly high elevations requiring maintenance crews to replace.
Ballasts used with these lamps historically are designed to optimize characteristics of power delivered to the lamps when the lamps are new having an initial impedance. As the lamps age, the characteristics of the lamps change. Typically the impedance of the lamp changes and the power levels and efficiency deteriorate. Ballasts have been designed to compensate for this impedance change, but there is still an efficiency loss in most ballasts.